Over the past several years local building codes throughout the country have increasingly required commercial and residential overhead garage door installations to be able to sustain higher and higher wind loads. This has been especially true for those counties in South Florida such as Dade County as well as other coastal regions where the threat of hurricane force wind is always a factor in determining structural safety. Generally speaking a garage door's ability to sustain wind load is directly related to the type and strength of the stiffener typically installed on the inside of the door. The current approach within the industry to meet these higher requirements has ranged from increasing the thickness of conventional stiffeners, to increasing the depth of the conventional stiffener designs as well as yield strength of the material used in making the stiffeners. In order to appreciate the uniqueness and novelty of the current invention, a better understanding of the current state of the art in addressing the above building requirements follows.
The first and most common approach taken by the industry in addressing the higher requirements has been to make the conventional stiffener out of heavier gauge material. Since the force that a wind exerts on a garage door generally increases with the square of the wind velocity, manufacturers using this approach have had to increase material thickness proportionally. These traditional stiffener designs include the C-channel stiffener as well as the hat-shaped stiffener. Heavier gauges such as 0.055 inch min. (17 gauge) to 0.070 inch min. (15 gauge) material are now common. The use of thicker material has not only lead to greater cost for garage door manufacturers and consumers but also as will be shown, has had the effect of creating other major problems simultaneously.
The garage door including any stiffeners is a system of parts interacting with each other as they are acted upon by wind load. Currently, residential and commercial overhead doors are typically constructed using steel skin with foam core assemblies, or using composites or wood. These are structures of marginal stiffness. These doors are typically supported by metal stiffeners to provide greater support as the door system sustains forces applied by the wind. However, an incompatibility occurs when stiffer sections i.e. stiffeners made of 0.055 inch min. to 0.070 inch min. material are joined or fastened to thinner less stiff sections i.e. steel skin doors made of 0.023 inch min. to 0.038 inch min. material. The area where these two sections are joined is an area of load transfer and thus of stress. The reason is that the stiffer section (i.e. the stiffener) resists conforming to the deformation of the less stiff section (i.e. the door) as wind load is increased. The result is that one part of the system (the door) tries to slide relative to another part of the system (the stiffener). This results in early failure caused by buckling of the door skin. This is due to in-plane compressive loads that result from the constraint that the stiffener imposes on the adjacent door skin as the door bends. Because of the increased stress at the joining area, manufacturers have been forced to modify parts of the garage door to offset this effect. For example, because the use of heavier stiffeners increases the shear load through the fasteners, especially on the outer extremes of the door width (near the rollers), heavier door panel end stiles have had to be introduced. Still another approach to alleviate the problem has been to use clips instead of threaded fasteners. This has been implemented in an attempt to reduce the high in-plane compressive stresses that the heavy stiffeners impose on the door skin. However, this approach is undesirable because by permitting sliding, it reduces the ability of the stiffener and door to act as a single system. This in turn reduces the total bending stiffness of the system and thus the effectiveness of the stiffener, since they now act more like independent components. This approach requires still heavier stiffeners, since the stiffener efficiency is greatly reduced when it acts as an independent component rather than as part of a system. Another drawback to clipping is that it requires substantially more parts and installation time.
The second approach generally taken by the industry is to make the current hat-shaped and C-channel stiffeners deeper and out of thinner yet higher yield strength material. This offers the advantage of reducing in-plane stress as noted above while at the same time increasing bending stiffness due to the deeper configuration. However this approach has major disadvantages.
First, the thinner material used in traditional stiffener configurations make these stiffener sections more susceptible to edge stress concentration. The conventional C-channel, and hat-shaped stiffeners have a "blade edge". This edge is very susceptible to imperfections in the sheet metal along this edge as well as to damage during manufacture, shipping/handling and installation. These imperfections along the blade edge become stress concentration points or focal points at which failure of the stiffener can initiate. A more detailed description of this failure initiation follows.
Even the most perfect, smooth edge of the conventional stiffener will experience a very localized point of high stress gradient due to the characteristic edge stress concentration associated with open sections under bending loads. Thus, initiation of an edge "bulge" or "crimp" on a perfect smooth edge is nothing more than the creation of an edge imperfection that is large enough to grow or "propagate" easily. It is significant that this stress concentration may be made worse by the presence of any relatively small local edge imperfections, even those on the order of size of the thickness of the stiffener material itself.
These imperfections near the edge can be in the form of edge notches, waviness (in-plane or out-of-plane), local thickness variations, local residual stress variations, or variations in material yield strength. Where multiple imperfections occur together, they may all compound together to further increase the stress concentration effect, and thus lower the wind load level at which failure is initiated. Thus, the existence of any edge imperfections in a conventional stiffener has the effect of enhancing an already established process of failure initiation.
Second, all the above conventional stiffeners, when manufactured out of relatively thin sheet metal are more susceptible to buckling due to the reduced thickness. Buckling is an instability in a part of the stiffener associated with local compressive or shear stresses. Buckling can precipitate section failure of the stiffener. This in turn causes a stress concentration in the adjacent door skin near the buckled stiffener section which causes the door to fail.
Finally, some thinner conventional stiffeners can experience "rolling" when placed under load. Rolling is when the shear stresses within the stiffener result in a net torque about the centroid of the thin walled cross-section thus causing the cross-section to twist possibly making the stiffener unstable. Another cause of rolling is the curvature of the door itself under wind load that is imposed upon the stiffener. Manufacturers have increased the cross-sectional length of the flange furthest from the door of the conventional C-channel stiffener trying to solve the rolling problem but were met with only marginal improvement. This is because the increased flange length had the simultaneous effect of increasing the distance from the centroid to the shear center of the channel. Additionally, increasing the cross-sectional flange length caused difficulty in accessing the fasteners used in mounting the C-channel to the door.
Because of the higher wind load requirements of local building codes and the problem of the fastening of relatively thick sections to sections relatively less thick, there is a need within the industry today for a new stiffener configuration that can address all of the above mentioned drawbacks and short comings of the present state of the art, is suitable for use with substantially all standardized overhead doors, and can be made on a cost effective basis.